1. Field of the Invention
Precision interferometer measurements, as are, for example, needed for measurements on masks and wafers in the semiconductor industry, have a resolution of well below the laser wavelength. To this end, the phase shift is electronically interpolated. This creates errors which lie far beyond the resolution of the measuring arrangement and occur periodically.
2. Description of the Related Art
Commercial high-resolution interferometers all work using the heterodyne principle, the two frequency components used being split by different polarizations. Even here, a periodic error occurs along the distance measured, but this is less due to the inaccuracy of the electronic interpolation, rather it has its basis in mechanical stresses in the components through which the laser beam passes. These mechanical stresses produce stress birefringence in the optical components which interfere with the ideal polarization of the two beam components relative to one another.
Periodic errors are just some of a whole range of errors which considerably impair the resolution and reproduceability of high-resolution interferometers. Examples of known causes of error include the fluctuating wavelength of the laser, electronic errors, optical errors and atmospheric fluctuations in the surroundings of the measuring arrangement. Since, in recent years, it has been possible to increase the resolution of interferometers to a great extent, great inroads are also being made into the elimination of individual errors and their causes.
For example, U.S. Pat. No. 5,469,260 A describes a high-resolution interferometer as part of a device for measuring position, in which by substantial enclosure of the beam paths and controlled supply of gas with known temperature and composition, the effect of air-pressure fluctuations and short-term temperature changes on the result of the measurement is minimized. Periodic errors are, however, not addressed there.
The problem of periodic errors in interferometer measurements has, however, been known for a long time. The manufacturers of commercial interferometers have also dealt with the question of periodic errors in interferometer measurements. This problem has also been dealt with in the article "Nonlinearity in Interferometer Measurements" by Robert C. Quenelle, Hewlett Packard Journal, April 1983, page 10. Even here, the peak values of the periodic error are indicated with an order of magnitude of 5 nm. It has hitherto not been possible to correct this error. However, there was neither any great need to carry out such correction, since hitherto known precision interferometer arrangements still had resolving powers above this 5 nm limit and the periodic error could therefore be classed as negligible.
The object of DE 40 31 291 A1 was to improve the interpolation or, alternatively, to reduce systematic periodic errors which occur through optical polarization perturbations. To this end, a heterodyne double interferometer is provided in which the beam path is split and the measurement signals are detected separately. In order to make it possible to orient the polarization directions of the two elementary beams ideally orthogonally to one another, adjustable analyzers are arranged in front of the two measuring detectors. In this case it is possible to compensate for the periodic errors with an orthogonal arrangement of the analyzers, or alternatively, with a parallel arrangement of the transmission directions of the analyzers, increased resolution of the measurement can be achieved. The inaccuracy in the measurement of the heterodyne double interferometer can therefore be improved to the submicrometer range with this arrangement.
A disadvantage with the previously known measuring arrangement is that considerable changes to the interferometer structure compared with known structures are needed. Indeed, the additional incorporation of a beam splitter, of an additional detector for the two elementary beams and of two analyzers in existing interferometer structures is not straightforward merely on the grounds of space. Furthermore, periodic errors which result from the electronic interpolation remain unaffected. Therefore, using this method, the inaccuracy of the measurement can merely be improved to the submicrometer range.
New interferometer arrangements for ultraprecision measurements have since reached a resolving power of the order of magnitude of below one nanometer, so that there is a need to eliminate the periodic error as far as possible.